Long life cathode for electron tubes



1940- D. D. KNOWLES ET'AL 2,139,636

LONG LIFE CATHODE FOR ELECTRON TUBES Filed Nov. 27, 1937 WITNESSES:INVENTORS flewey D. Knowles and t George Evans.

A ATTORNEY Patented Feb. 6, 1940 LONG LIFE CATHODE FOR ELECTRON TUBESDewey D. Knowles, Wilkinsburg, and George S.

Evans, Turtle Creek,

Pa., assignors to Westinghouse Electric & Manufacturing Company, EastPittsburgh, Pa., a corporation of Pennsylvania Application November 27,1937, Serial No. 176,852

2 Claims.

The invention relates to cathodes, and especially to cathodes forgaseous discharge devices. An object of the invention is to provide acathode having a reserve supply of emissive 5 material to supplement theregular supply,

especially when the regular supply is not sumcient for the load currentbeing conducted through the discharge device.

Other objects and advantages of the invention will be apparent from thefollowing description and drawing, in which the drawing shows across-section through a tube disclosing a preferred cathode therein.

The electron emission from any of the thin film cathodes and from thelatest type of oxidecoated cathodes is known to be either directly orindirectly due to free barium in or on the surface of the cathode. Thisbarium is ordinarily made available by difiusion from the core materialeither by decomposition of the coating due to heat or by electrolysis.The life of such a cathode is, therefore, limited to the length of timethat this barium supply can be maintained. There are various factorswhich limit the amount of barium supply that can be provided in theordinary type of oxide-coated cathode structure.-

If a thick coating of oxide is used to obtain more barium, theresistance becomes too high to be practical.

The invention concerns the idea of providing a reservoir of tree bariumthat will provide an auxiliary supply of electrons especially when theusual supply is not sumcient.

In the drawing, we have disclosed a tube I0 35 having usually a gaseousfilling ll therein and an anode l2.

The cathode comprises a casing 28, preferably in the form of a thimbleof metal, such as nickel or iron, having at the upper end a porousmetallic block 24 having an opening 25. This block or top is preferablyof sintered tungsten or of a .metal having a porosity of the order ofsintered tungsten. A piece of metallic barium 2| is placed in the bottomof this casing or thimble. The porous top 20 has electron-emissivematerial in its pores such as barium and strontium oxide and a suitableheater 22 maintains the porous top or block at a suitable electronemitting temperature which may be of the order or 900 C.

It will be noted that the top 24 has an opening 25 therein exposing thefree barium 2| to possible bombardment. It is desired that a temperatureof approximately 400 C. be applied to the free barium 2| in the bottomof the casing while the top of the casing is operated at a temperatureof approximately 900 C. If the emission of the electrons from the top ofthe crater cathode is sufficiently good, the voltage drop will be lessand not many positive ions will project to the bottom of the casing andbombard the barium. If the emission decreases, however, the drop willincrease and consequently, the number of high speed ions striking thebarium pellet will increase. These ions sputter of! the barium andreplenish the supply of barium at the crater opening 25. The advantageof the cathode structure shown is that the adjustment of additionalelectron emission is always instantaneous because there is no delaycaused by the necessity of some part changing its temperature.

Many modifications may be made in the form, number and arrangement ofthe various elements disclosed and their application to other types oftubes. Accordingly, no limitations should be imposed upon the followingclaims except as necessitated by the prior art.

We claim as our invention:

1. Cathode structure comprising a casing, a top on said casing having aporosity of the order of sintered tungsten, electron emitting materialin said porous top, and barium within said casing, said top having anopening therethrough whereby said barium may be bombarded by ions fromoutside said casing having sufllcient velocity.

2. A cathode for an electrical discharge device comprising a casingenclosed except on the top side, the top side beingof material havingpores of the order of those characterizing sintered tungsten and havingan opening through it which has a diameter many times larger than saidpores, a supply of an alkaline-earth, metal within said casingpositioned so as to be bombarded by ions entering said casing throughsaid opening.

DEWEY D. KNOWLES. GEORGE S. EVANS.

